Electrodeposition of chromium

ABSTRACT

AN IMPROVED METHOD AND AN IMPROVED ELECTROLYTE SOLUTION FOR PROVIDING A CHROMIUM COATING ON A METAL SUBSTRATE. THE METAL COATINGS ARE PROVIDED BY ELECTRODEPOSITIONS FROM SOLUTIONS COMPRISING WATER, DIMETHYLFORMAMIDE, TRIVALENT CHROMIUM, HALIDE IONS, AMMONIUM IONS, AND A CATIONIC SURFACTANT SELECTED FROM THE GROUP CONSISTING OF TETRAALKYLAMMONIUM SALTS AND SUBSTITUTED IMIDAZOLINE.

United States Patent U.S. Cl. 204-51 13 Claims ABSTRACT OF THE DISCLOSURE An improved method and an improved electrolyte solution for providing a chromium coating on a metal substrate. The metal coatings are provided by electrodepositions from solutions comprising water, dimethylformamide, trivalent chromium ions, halide ions, ammoniumions, and a cationic surfactant selected from the group consisting of tetraalkylammonium salts. and substituted imidazoline.

This application is directed to the electrodeposition of chromium and more particularly to the use of a surfactant as an ingredient in an improved electrolyte solution.

In recent times substantial improvements have been made in the appearance and the performance of electrodeposited metal coatings, particularly in connection with the deposition of chromiumJH'ow'eve'r, substantial problem and disadvantages still exist in the presently available processes. For example, conventionalchromic acid baths deposit the metal at lowcathode efficiency and provide coatings having poor covering. Moreover, conventional baths often have poor throwing power. Chromium deposits from conventional baths have been found to be in a highly stressed condition so that they are sub ect to crack development when the thickness reaches a few hundredths of a milli-inch. Higher current efiiciencies, which may approach 40%, can be obtained with aqueous solutions of trivalent compounds but the deposits from such solutions are satisfactory'in appearancebnly when deposited over a very narrow range of pH values. The large amount of hydrogen gas evolved during deposition tends to cause substantial changes in the pH of the solution in the vicinity of the cathode, thus making pH control difficult and restricting the permissible current density range which can be used.

It is recognized by those in the art that considerable economic advantage could be achieved if the current efficiency could be increased. This advantage is particularly attractive with respect to chromium plating baths. It is also recognized that the evolution of hydrogen from the cathode during deposition of the metal is a serious problem and that coatings of improved corrosion resistance and appearance can be provided if the evolution of hydrogen can be controlled. In the past, experimentation with the use of modified chromic acid and trivalent chromium baths has shown little prospect of improvement in the quality of the deposit without a concomitant reduction in current efiiciency.

As an improvement over chromium plating from conventional aqueous chromic acid baths, a method has been developed whereby chromium deposits, having greatly improved corrosion resistance and exhibiting a very good appearance, can be provided by electrolytic techniques through the use of a plating bath comprising chromium ions, particularly trivalent chromium ions, and a homogeneous mixture of water and a dipolar organic solvent,

preferably aprotic, such as dimethylformamide.

The ratio by volume of water to dimethylformamide may range from 10:90 to 95:5 but preferably between 60:40 and :10. These lower concentrations of dimethylformamide make possible a reduction in the cost of the plating bath without necessarily impairing the plating performance.

Solutions containing trivalent metal salts, such as chromic chloride, in combination with organic dipolar solvents and water, may be unstable with respect to composition and plating performance. The physical characteristics of the solution, e.g., color, pH, and viscosity, have been found to change during storage and the color and adhesion of successive metal deposits from the same bath may deteriorate. Moreover, plating baths containing low concentrations of Water have a relatively low conductivity, while solutions with higher concentrations of water tend to evolve excessive amounts of hydrogen at the cathode during plating. It has been found that the presence of an ammonium salt improves the stability of the solution and reduces the tendency of the bath to evolve hydrogen at high water content. Moreover the presence of ammonium salts has been found to significantly reduce the effect of changes in pH on the lower limiting plating current density. The ammonium ion should be present at a concentration of at least about 0.2 M and preferably from about 0.6 to about 1 M. At lower concentrations of dimethylformamide, it may be preferable to increase the ammonium ion concentration to above 1 M.

A prerequisite of successful chromium plating from a dipolar organic solvent is that the trivalent chromium ions shall form small moderately stable complexes with the solvent molecule. If there is no complex formation, then the chromic salts are unlikely to be sufficiently soluble. If the complexes formed are excessively stable, then electrodeposition may be diflicult. It is believed that the highly electronegative oxygen atoms which characterize the dipolar organic compounds of this invention may act as covalent links in the formation of complexes between the chromic ions and the organic molecules. Such solutions by themselves do not, however, give smooth coherent metallic deposits. The addition of water probably generates the polynuclear olated and oxalated chromic species usually found in solutions of trivalent chromium compounds, which again do not readily give good chromium deposits. The effect of the ammonium ion may be attributed to its structure-disordering properties simplifying the nature of the trivalent chromium in solution, possibly with the formation of mononuclear Cr +DMF-H O complexes. The watermay be prevented from showing its full protic tendencies by reason of the formation of complexes with the dipolar organic molecules. The halogen ions may be partly solvated, although the ready evolution of chlorine at the anode indicates solvation is not complete.

The pH of the solution should be from 1 to 3.5 and preferably about 2. If the pH is too low, hydrogen tends to be evolved at the cathode in preference to chromium. If the pH is too high, basic chromium compounds are liable to precipitate out. The pH can be adjusted by the use ofhydrochloric acid or sodium hydroxide as required.

The current efficiency of the solution may be improved by the addition of boric acid, preferably to a concentration of at least 0.1 M. Boric acid is not normally soluble to the extent of more than about 0.2 M.

In addition to the ammonium ions mentioned above, electroplating baths of the present invention contain a sodium halide which has been found to increase the plating range and current efliciency. Sodium halides are also beneficial in that they enhance the covering power of the bath. It is preferred that the sodium halide be present in a concentration of at least about 0.8 M. Again, however, if lower concentrations of dimethylformamide are to be used, it may be advantageous to increase the halide ion concentration above 5 M, for example from 7 to 10 M.

As is pointed out above, it is generally preferable for reasons of economy to utilize solutions having lower DMF chain alcohol of the general formula C H OH and sodium salt of a carboxylic acid of the general formula 5 concentrations and hence a greater proportion of water. CnHznOCn'Hm'COONa With such solutions, however, certain problems are encountere R, 15 an alkyl having 9 to 17 carbon atoms, R is an The inclusion of large amounts of chloride and amalkylefle havlllg 1 t 4 q a X 1 s a halogen monium ions increases the expense, the plating efficiency 10 Seleeted' from the fp eonslstlng bromine, ehloflne and current density range may not be very good, and and lodlne and are 1 to the deposits tend to be dark and in other ways less at- The surfactant should be present 111 the solution in a tractive than those derived from baths with a higher eolleelltfatlell of at least 5 P-P- and P e l at least organic content. p.p.m. Below 5 p.p.m., little or no effect is discernible. In accordance i the present invention, h The disadvantages of non-ionic and anionic surfactants blems are overcome by adding a surfactant t0 the plating beglll to become dlseemlble eoneemratlons above solution. Although it has been found that a surfactant P-P- There pp to lime or no advantage m can improve the plating range at any proportion of using surfactant concentrations above 20 p.p.m., and the Water to dipolar organic solvent, it is particularly d. use of unnecessary surfactant is undesirable as it causes vantageous at proportions of water to dipolar organic 20 the bath However, as il be aPPfeelated, 10 f solvent of greater than 60:40 by volume where it enables 20 P'P- 15 a y Small amour}! P surfactam- It 15, bright chromium deposits to be obtained under suitable therefore, p le use a eatlome surfaetanb where conditions instead of the matt plate produced in'the the aeeldemfll addltlon 9 as to the Plamlg bath absence of surfactant. Although bright chromium deposits does P e f p are obtainable from solutions containing as little as 250 The lnventlol'l 15 illustrated y the fellowmg p e g./l. of DMF in the presence of a surfactant, it is pre* The Platlng baths p y all eolltalned the follOWlIlg ferred that the ratio of water to solvent range between components: 60.40 and 75:25. The electrolyte will function at lower Cbmmic id 1 4 DMF concentrations but the plating range becomes pro- Ammonium chloride Variable gressively smaller. At higher DMF concentrations there "Sodium chloride M 0.8 is no reduction in plating performance but the operating Boric acid ..g./l 2

TABLE NH'H concen- Water:DMF, Concen- Plating tration, weight tration, range,

ratio Surfactant p.p.m. pH AJm. Comments 1.0 60:40 1.2 2,250-160 An average result. 1.0 60:40 Nalauryl sulphate 10 1.2 2,250-140 Asllglit iimgrmrtient in range. Black S C S n B 1.0 60:40 Cetyl trimethyl ammonium bromide.- 10 1.1 2,250-130 Ag bd result. 1.0 60:40 .-...d0 15 1.2 2,250120 Very good result. 1.0 60:40 do 30 1.2 2, 250430 Goodt luttlisfiight frosting at high cur- [811 0 1.0 60:40 Substltutedimidazoline 10 1.2 2,250-130 Good result. 1.5 70:30 1.1 2,250-180 Anaverageresult. 1.5 70:30 Cetyl tn'methyl ammonium bromide, 10 1.1 2,250-130 As goodasthe 60:40bath. 1.0 76:24 1.1 2, Noticeablelosslnplatlngrange. 1.0 76:24 Cetyl trimethyl ammonium bromide 10 1.1 2.250475 Do.

1 Miranol L2M-SF manufactured by Venture Chemicals Lt'd., England, the major constituent of which is N C H) L cell voltage, as well as the cost of the electrolyte, increases.

The surfactant may be cationic, anionic, amphoteric or non-ionic, but cationic surfactants are preferred. Nonionic surfactants tend to give rise to dull deposits, particularly when used in higher concentrations. Anionic surfactants such as Na lauryl sulphate, again particularly at higher concentrations, tend to give rise to black specks on the metal deposits.

Cationic surfactants that may be used according to the invention are tetraalkylammonium salt of the general formula /f\ R: R4 X and substituted imidazoline of the general formula wherein R is an alkyl having 8 to 18 carbon atoms, R,, R and R are alkyls having from 1 to 2 carbon atoms,

R is selected from the group consisting of aliphatic short chain alcohol having up to 6 carbon atoms of the general formula C H OH and sodium salt of a carboxylic acid of the general formula C H OC 'H ,CO'ONa, R is an alkyl having 9 to 17 carbon atoms, R, is an alkyle ne having 1 to 4 carbon atoms, and X is a halogen selected from the group consisting of bromine, chlorine and iodine, and n and n are 1 to 6, the proportion of water to dipolar aprotic solvent being from about :90 to about 90:10.

2. An electrolyte solution according to claim 1 having ammonium ions in a concentration of at least 0.2 M and halogen ions in a concentration at least 0.8 M.

3. An electrolyte solution according to claim 2 wherein the dipolar organic solvent is dimethylformamide.

4. An electrolyte solution according to claim 3 wherein the proportion of water to dipolar aprotic solvent is from about 60:40 to about 75:25.

5. An electrolyte solution according to claim 4 having halogen ions in a concentration of at least 5 M.

6. -An electrolyte solution according to claim 4 having ammonium ions in a concentration of at least 1 M.

7. An electrolyte solution according to claim 3 wherein R R and R are methyl groups.

8. An electrolyte solution according to claim 7 wherein the surfactant is cetyl trimethyl ammonium bromide.

9. An electrolyte solution according to claim 3 having between about 5 p.p.m. and about 20 p.p.m. of said surfactant.

10. An electrolyte solution according to claim 3 having said surfactant in a concentration of between 5 p.p.m. and such amount as causes foaming of the electrolyte solution.

11. An electrolyte solution according to claim 3 having ammonium ions in a concentration of between about 0.6 M and about 1 M.

12. An electrolyte solution according to claim 3 wherein the halogen is chlorine.

13. An electrolyte solution according to claim 3 having a pH between about 1 and 3.5.

References Cited UNITED STATES PATENTS 2,750,335 6/1956 Brown et al. 204-51 3,341,434 9/1967 Passal 204-51 3,423,297 6/ 1969 Van der Horst 204-51 3,706,641 12/1972 Huba et al. 204-51 FOREIGN PATENTS 1,144,913 3/1969 Great Britain 204-51 262,033 6/1970 U.S.S.R 204-51 GERALD L. KAPLAN, Primary Examiner 

